This invention relates to waveguides, including coplanar waveguides, formed within electrically-conductive sheets disposed on opposite surfaces of a dielectric substrate and, more particularly, to a system for coupling electromagnetic power to antenna radiators formed within a conductive sheet, the power being coupled from beneath the sheet to avoid the presence of coupling elements within a path of radiation transmission.
Circuit boards comprising a dielectric substrate with opposed surfaces covered by metallic, electrically-conductive sheets are often used for construction of waveguides for conducting electromagnetic power among electronic components, such as radiators of an antenna, filters, phase shifters, and other signal processing elements.
There are three forms of such circuit boards. One form, known as stripline, comprises a laminated structure of three electrically conductive sheet spaced apart by two dielectric substrates. The middle sheet is etched to form strip conductors which cooperate with the outer sheets, which serve as ground planes, to transmit a TEM (transverse electromagnetic) wave. A second form of the circuit board, known as microstrip, is also provided as a laminated structure, but is simpler than the stripline in that there are only two sheets of electrically conductive material, the two sheets being spaced apart by a single dielectric substrate. One of the sheets is etched to provide strip conductors which in cooperation with the other sheet, which serves as a ground plane, supports a TEM wave. The third form of circuit board is provided with a coplanar waveguide, and comprises two sheets of electrically conductive material spaced apart by a dielectric substrate. The coplanar waveguide is formed completely within one of the sheets and is constructed as a pair of parallel slots etched within a conductive sheet, the two slots defining a central strip conductor. The central strip conductor cooperates with outer edges of the slot to support a TEM wave.
The microstrip and the coplanar waveguide structures are of particular interest herein because of their utility in interconnecting microwave components by use of a circuit board, which may be employed to support these components. Also, their relatively simple structure of a single dielectric layer, or substrate, with covering of metallic sheet permits interconnection with a variety of physical shapes of electronic components, particularly for the excitation of radiators of an array antenna. This permits greater flexibility in the layout of the components on a circuit board.
In the use of the circuit boards, it is frequently necessary to couple a portion of the power from one waveguide to another waveguide for combining signals such as, for example, in the case of a Butler matrix for distributing electromagnetic signals among elements of a phased array antenna. The capability for coupling electromagnetic signals between waveguides is particularly important in situations wherein power is to be coupled through a circuit board between a waveguide on one side to a circuit component, such as an antenna element, on the opposite side of the board. Heretofore, such coupling has been accomplished by use of a feed-through connector with appropriate impedance matching structures. Alternatively, power has been coupled to antenna elements by coupling elements which are located within the radiating aperture of an antenna element with adverse influence on the radiation pattern.
A problem arises in the deployment of coupling elements within the radiating aperture of an antenna element in that the design of the antenna element is made more complex by the need to diminish any adverse effects on the radiation pattern due to the presence of a coupling element in front of the radiating element. A problem also arises in the use of a feed-through connector for energizing an antenna element from behind the element in that additional manufacturing steps are required. For example, a microstrip waveguide and a coplanar waveguide can be manufactured by photolithography including an etching of the waveguide structure in a metallic sheet. In order to provide the feed-through connector, it is necessary to drill a hole through the dielectric substrate, and then to establish an electrically conducting path through the drilled hole. In addition, a feed-through connector may also entail the use of additional impedance-matching structures to avoid unwanted reflections from a discontinuity in the waveguide presented by the feed-through connector.